We need an ATG Mystery Set

This would be a blast!! You could actually create 2 ATG Mystery Sets...one for players before the DH (1973) and one for players after the DH (1973)

You would have 5 cards for every player...
Card#1 - Rookie (First full season)
Card#2 - Best season
Card#3 - 5th best season (or some variation)
Card#4 - Closest to career averages
Card#5 - Weakest (worse full season)

Example : Albert Pujols
Card#1 - 2001 AVG=.329 OBP=.403 SLG=.610 OPS= 1.013
Card#2 - 2009 .327 .443 .658 1.101
Card#3 - 2005 .330 .430 .609 1.039
Card#4 - 2002 .314 .394 .561 .955
Card#5 - 2017 .241 .286 .386 .672

Example #2 : Frank Robinson
Card#1 - 1956 AVG=.290 OBP=.379 SLG=.558 OPS= .936
Card#2 - 1966 .316 .410 .637 1.047
Card#3 - 1967 .311 .403 .576 .979
Card#4 - 1965 .296 .386 .540 .925
Card#5 - 1974 .245 .367 .453 .820

you may have to be creative and add in career journeyman players like Craig Counsell and Mark McLemore and Biff Pocoroba ...

what do you think?

Yes,yes and more yes. I personally don't understand why every player who has more than 5 strato cards doesn't have the option of being in a mystery league. I like your idea but would be fine with the cards being completely random if the player has 6 or more cards.
I understand there could be some issues with playing certain season/periods and guys that are often used now not having 5 cards but I think that Rosey's add list would help to fill those holes even quicker than strat putting paid hours on it.
Man, I wish they would just open source this or at least give a few savants (imagine there is more than one in strato) free access of a copy to see what they could come up with and fix in a short period of time. I understand not wanting to tweak with something that is working, especially when you don't fully understand why it's working or what your changes may cause -- and wish I knew the terminology better but would think experimenting with backup copies of the engine could yield some extremely valuable information. After all, how hard is it to change a variable and let it run along the current engine and current seasons and quickly be able to tell, narrow down and move closer to how those changes most likely affected the results?

This is a GREAT idea...

But it would be "too much" work for them.

We can't even get cards entered into ATG, let alone have something made like this.

I agree great idea. However they cannot even bring back something they have already the year 1969. Why?